The present invention relates to torque transmission couplings specifically as applied to marine propellers and the transfer of torque from a propeller shaft or an engine drive shaft to the propeller for rotating the propeller to create thrust and propel a vessel.
Many methods are known for accomplishing the task of transferring torque from a propeller shaft to a propeller. Perhaps the simplest is a direct mechanical connection between the shaft and the hub such as by welding, by cross pinning the propeller to the shaft, or by using a splined shaft arrangement for example. However, direct mechanical connection between a propeller and a propeller shaft will result in significant damage to any one of or all of the components of the propulsion system if the rotation of the driven propeller is abruptly interrupted by striking an underwater obstacle or the like. Such an occurrence develops severe torque loading of the propulsion system and can damage one or all of the blades of the propeller; destroy the propeller hub or the connection between the propeller and the propeller shaft; overload and burn propeller shafting bearings; overload and break transmission and reversing gearing used in the propulsion system, including clutch mechanisms for engaging and disengaging the engine from the propeller shaft; and overtorque the engine, resulting in a variety of damage.
An early remedy to minimize the kind and extent of damage resulting from a propeller striking an obstacle relates to cross pinning the propeller to the propeller shaft, but by using a shear pin. The propeller is journaled for slip fit engagement with the propeller shaft and is precluded from such slippage by the insertion of a shearable cross pin which in its simplest form extends through the propeller hub and the propeller shaft. However, this remedy also has its problems insofar as a shear pin may fail to shear at the design torque loading. Even if the pin does properly shear, it then may potentially scour the journaled surface of the propeller hub, causing the propeller to cease to the propeller shaft. Another potential problem with using a shear pin relates to inattentive or inexperienced operators who may not recognize a failure situation after the shear pin has done its job, namely shear, and who will allow the propeller shaft to spin excessively inside the disconnected propeller, again causing the propeller to cease to the propeller shaft. Finally, a shear pin may all too often and easily be replaced with any piece of metal of suitable size, commonly a nail or steel rod for example, which will typically have shear characteristics far exceeding the design shear of the appropriate shear pin. This unwitting substitution results in a mechanical connection between the propeller and propeller shaft of the variety discussed above.
No safety propulsion measure, whereby some degree of residual torque transfer to the propeller remains after the propeller strikes an object, is designed into the propeller mounting systems discussed above. Thus, the vessel is commonly rendered nonoperational and must rely upon other sources for passage to a safe harbor where repairs may be effected.
A common contemporary resolution to the above problems is what might be called a rubber insert or bushing propeller mount. This typically includes a tubular, hard rubber bushing which circumscribes and is vulcanized to a centrally located, metal propeller mounting sleeve. The bushing and sleeve assembly is force fit into a center opening in the propeller hub by application of large forces to the hub and the bushing and sleeve assembly. The propeller mounting sleeve is mechanically connected with the propeller shaft and the rubber insert provides a slip clutch effect between the propeller hub and the propeller mounting sleeve insofar as the propeller shaft and mounting sleeve are allowed to rotate or slip relative to the propeller with severe torque loading of the propeller. Torque transfer to some degree may resume in this propeller mount after the impact or severe torque condition is removed. Thus, this variety of propeller mount provides a degree of damage safety and residual torque transfer subsequent to release.
However, this mount also has various problems. One problem is that the rubber bushing will typically harden with age. Another is that the bushing will also adhere to the inner cylindrical wall of the propeller hub with age. Either of these conditions significantly increases the torque value at which the slip clutch effect will occur. Thus, the damage safety feature of the rubber bushing mount diminishes as the propeller, specifically the rubber bushing, ages.
Another difficulty with the rubber bushing propeller mount is that the amount of torque transferable through the bushing will typically change significantly once slippage occurs. Sometimes the torque transfer capacity of the mount will be greatly diminished, more frequently the torque transfer capacity of the mount will be greatly increased and result in diminished safety for subsequent impacts.
Yet another problem with the rubber bushing propeller mount is found in the repair of this propeller mount. A mechanical press of some sort is required to disassemble the rubber bushing propeller mount and a force as high as sixteen tons is commonly required to press out such rubber bushings from a propeller hub. This amount of force is often unobtainable in many repair shops. Further, this level of force may easily damage and destroy a propeller which is under repair. If the old or damaged rubber bushing is removed from the propeller hub, then a new bushing must be pressed into place in the propeller hub and this procedure again requires a mechanical press capable of large forces, as high as twelve tons.